Recent experimental and theoretical developments in surface science showed how hot electrons cause heating of the vibration of molecules or atoms adsorbed on a catalyst surface. The energy of hot electrons is defined as electrons with effective temperatures between 600 Kelvin and 60,000 Kelvin, which means equivalent energies between 0.05 and approximately 5 eV, or many times that of thermal energy at room temperature. 300 degrees Kelvin is 0.026 eV.
It has been discovered that hot electrons that diffuse to a catalyst metal surface interact strongly with the adsorbed surface chemicals, also called adsorbates, and can do so at a rate faster than the process of electrons thermalizing with the lattice of catalyst metal atoms. It has also been recently discovered that the adsorbates acquire vibrational energy when interacting with hot electrons from the catalyst surface. It has been further discovered that adsorbate vibrational energy strongly enhances the rate of chemical reactions, and in some cases enable reactions that do not occur by thermal means because of the activation energies or chemical thermodynamics involved. Hot electrons stimulate adsorbate chemical reactions on a catalyst surface. The reverse of this process has also been observed, where a surface chemical reaction resulted in the production of hot electrons.
The presence of hot electrons on the surface of the catalyst can cause a pseudo-thermal regime in which the surface vibrations of adsorbate molecules, either against themselves or against the catalyst, are in equilibrium with the temperature of the substrate hot electrons rather than with the physical temperature of the substrate itself. This means the vibrations can be at several thousand degrees while the catalyst is at ambient temperature. Hot electrons excite the adsorbate from the bottom of its adsorption well in a stepwise manner, and may even do so until it overcomes the adsorption barrier and hops to a neighboring potential well, reacts or desorbs.
The hot electron energy or frequency need not exactly match that of the adsorbates. The adsorbate excitation structure is generally very broad, being spread over many frequencies, and the mechanism is often via an electronic excited state. That is, when the adsorbate acquires an electron it transitions to an excited electronic state. Within a few tens of femtoseconds it begins to move outward away from the surface, and then releases the electron. The adsorbate now transitions back to a non-electronic excited state. However, it retains the extra energy given to it by the hot electron. As a result, the adsorbate is in a higher vibration state. The tens of femtosecond lifetime for the process causes a broadband resonance feature and hence permits an energy mismatch between hot electron and the receiving adsorbate energy levels. The substrate electron in effect deposits energy into a vibration mode of adsorbate reactant, such as the vibration of the atoms in the adsorbate reactant molecule or in the vibration of the adsorbate against the catalyst surface. This process can repeat itself many times, to the point where the adsorbate desorbs. In the literature this is called “Desorption Induced by (Multiple) Electronic Transitions,” Abbreviated DIMET or DIET. This is the stimulator process.
The generator process works in reverse. An adsorbate with energy in one of its vibration modes attracts and acquires a cold electron from the catalyst. This causes a transition where the adsorbate with attached electron then becomes a charged adsorbate specie in an excited electron state. Within femtoseconds this specie in the excited electron state decays and ejects an electron. This leaves the adsorbate reactant with less energy in its vibration mode and the electron with excess kinetic energy. The effect is that the energetically excited reactant on the surface of the catalyst gave a fraction of its energy to an electron in the catalyst. This is the generator process.
This generator or reverse process has been observed in laboratories. The detector in this observation used a short circuit Schottky diode to measure an electron flux directly generated by the surface reactions. The laboratory detector measured a current in a short circuit diode, which means the detector generated almost exactly zero power. However, the detector confirmed the existence of the generator mode. Both hot electrons and hot holes were observed, and with energies in excess of the Schottky barrier in silicon, which is of order 0.5 eV.
Hot electrons on a catalyst surface have been shown to accelerate reactions. Experiments with vibrationally excited Nitrogen Oxide (NO) molecules interacting with a copper (Cu) surface showed thousand-fold enhancement of surface reactivity. Up to near unit reaction probability was observed. In that work, neither reactant translational energy nor surface temperature had a strong effect on the reaction probability, confirming the efficacy of using hot electrons.
In another experiment, carbon monoxide (CO) was oxidized on a ruthenium surface. A 1.5 eV, 110 femtosecond laser pulse duration created the hot electrons. It was observed that sub-picosecond reactions of adsorbed O with CO to produce CO2 in a reaction that is energetically not possible at all without the hot electrons. This means if one uses thermal energy alone, CO will desorb without reacting.
The efficiency of such hot electrons to impart vibrational energy to just the adsorbates can approach 100%. Nearly 100% desorption of CO from a copper surface was observed. A three order of magnitude increase in reaction rate of NO with Cu was also observed.
This establishes the strong, two way energy transfer between hot electrons and excited adsorbate specie on a metal catalyst surface. The collection of observations leads to both an apparatus and method to couple the excitation structures of the adsorbate reactants adsorbed, chemisorbed or physisorbed on a catalyst surface to the excitation structure of a semiconductor diode in close proximity to the adsorbates.
The semiconductor diode excitation structure is rather simple, consisting of holes in the valence band and electrons in the conduction band. The excitation structure of the chemically reactive adsorbate-catalyst system is dominated by vibrations of the atoms and molecules with themselves and against the substrate, forming energy level bands, and the energy level bands due to electronic excited states of these specie, where the adsorbates may acquire a transient or permanent charge.
Coupling of these structures occurs mainly by two paths, either directly through the direct, typically ballistic transport of the hot carriers such as hot electrons or hot holes, between adsorbate and semiconductor, or by resonant tunneling of energy. Resonant tunneling couples the two structures through oscillating electric fields produced by the excitation structures in the semiconductor and adsorbate-catalyst system. The coupling is greatly enhanced when the frequencies of the excitations on either side are close to each other.
Hot electrons are the easiest excitation to work with. The current method of choice to produce and inject the hot electrons into a metal catalyst surface relies on a pulsed laser. The usual method to produce these hot electrons is to irradiate the surface of the metal with a short laser pulse, typically with pulse duration in the range of 50 to 1000 femtoseconds and with photon energies of 1 eV or greater (0.2 to 1.5 micron wavelength). The photons are adsorbed and produce electrons with energies between 0 eV and up to the photon energy, splitting the energy with a hot hole, and with hot electron energies averaging approximately half the incident photon energy. A laser, however, is one of the most expensive energy sources available.
A theory to use solid state metal-insulator-metal junctions to produce resonantly coupled, hot electrons has been proposed. The theoretical suggestion would produce resonance-assisted, hot-electron-induced femto-second chemical processing at surfaces. The energies relative to the catalyst Fermi level and associated with the metal-insulator junctions is higher than what is now known to be appropriate for surface resonances. No experiments using this theory are known at this time. No known mention of process reversibility has been claimed.
The use of a neutral semiconductor substrate as the injection mechanism into thin metal overlayers, with photons derived from a pulsed laser as the creator of hot carriers in the semiconductor, was also suggested in the literature. It was suggested that this could be an order of magnitude more efficient for stimulating gas-surface catalytic reactions than using the metal as the photon acceptor. It was suggested that using a semiconductor substrate, metal overlayer and catalyst device to produce hot electrons more efficiently with photons and inject them into a catalyst surface. However, process efficiency needed to render the approach useful was not adequately addressed. One must tailor the Schottky junction, the ohmic junction or the almost ohmic junction between the semiconductor and the metal so that the coupling of either hot carriers such as hot electrons or holes is electrically efficient, or so that the resonant tunneling is efficient. The proper use of resonant tunneling and resonance-assisted processes can be valuable components in a useful device and method.
A Schottky junction diode has been used in experiments for hot electron injection into solutions. One of the co-authors of that work suggests that they did not achieve the success they wanted because the surface states associated with the electrolyte cooled the electrons. A catalyst electrode Schottky junction made of n-silicon and platinum metal was used to inject electrons into a reactive electrolyte solution. The platinum thickness was varied from less than the mean free path to several times thicker than the mean free path of hot electrons in platinum. They achieved some success, and also suffered severe problems with interactions between hot electrons and electrolyte. Flooding the surface with liquid electrolyte destroys the effectiveness of hot electrons. Metal-oxide junction surface states have been an unsolved problem with this approach, where liquids flood the reactive surface.
It is now known that outer layers, away from the catalyst surface, of multiple layers of adsorbates that accumulate on the metal-liquid interface trap hot electrons as “polaritons” and render them less useful as a source of prompt reaction stimulators or as generators of excitations. The efficacy of a semiconductor substrate under a metal and catalyst reactive surface is a valuable element. A semiconductor diode is a critical element.
Implicit in all the observations is the efficiency of pulsed operation. In the case of a reaction stimulator, the duration of the pulses generating hot electrons is less than the time associated with electron thermalization with the lattice. In the case of a generator, the sudden burst of chemical reactions causes a flood of hot electrons on the catalyst surface. This in turn causes a flood of electrons in the conduction band of the semiconductor substrate collecting those hot electrons. A sufficiently short burst causes the number of generated electrons to exceed the thermally occurring short circuit electrons, thereby increasing the efficiency of the generation of electricity.
Missing in the public domain are methods to tailor the surface of the catalyst to enhance resonant tunneling, to enhance the activation of selected energy bands, to enhance the probability of desired energy transitions, or to enhance the selected reaction pathways.